Toll-like receptors (TLRs) are type I membrane glycoproteins that are key receptors in innate immunity. The 10 TLRs known in humans recognize different microbial antigens, and when activated by ligand binding, mediate rapid production of cytokines and chemokines. In addition to their role in host defense, TLRs play a role in cancer progression and development and cell protection.
TLR5 binds flagellin, a globular protein that arranges itself in a hollow cylinder to form the filament in bacterial flagella. Binding of flagellin to TLR5 initiates a cascade of pro-inflammatory molecules, notably NF-κB and its targets. TLR5 agonists derived from flagellin have been developed as therapies various diseases. However, these molecules may suffer from specific limitations, including for example, unsatisfactory binding and signaling. Additionally, many possible hosts already produce anti-flagellin antibodies that also target the TLR5 agonist derivatives, thereby clearing the therapeutics from the body and limiting their efficacy. Moreover, as intrinsically immunogenic bacterial proteins flagellin derivatives may possess disadvantageous antigenicity and immunogenicity, and therefore warrant improvement.